Mobile communication devices may be configured to provide voice communications as well as data communications with use of software applications. Examples of software applications used in these wireless devices include micro-browsers, address books, email clients, instant messaging (IM) clients, and wavetable instruments.
Such mobile devices may also have access to a plurality of services via the Internet. A mobile device may, for example, be used to browse web sites on the Internet, to transmit and receive graphics, and to execute streaming audio and/or video applications. The transfer of Internet content to and from wireless device is typically facilitated by the Wireless Application Protocol (WAP), which integrates the Internet and other networks with wireless network platforms. Such mobile devices may operate on a cellular network, on a wireless local area network (WLAN), or on both of these types of networks.
With respect to WLANs, the term “Wi-Fi” or Wireless Fidelity pertains to certain types of WLANs that use specifications in the Institute of Electrical and Electronics Engineers (“IEEE”) 802.11 family. In a WLAN, an access point (AP) is a station that transmits and receives data (sometimes referred to as a transceiver). The AP in an infrastructure BSS (“Basic Service Set”), or a client node acting as an AP in an Independent BSS, connects devices to other devices within the network. Such AP may also serve as the point of interconnection between the WLAN and a wired LAN. Each AP can serve multiple users within a defined WLAN area. As users move beyond the range of one AP (i.e. when they roam), they are automatically handed over to the next AP. Note that a small WLAN may only utilize a single AP or only a few APs. The number of APs required increases as a function of the number of network devices/users and the physical size of the network. The AP may be an IEEE 802.11 (i.e. WLAN) radio receiver/transmitter (or transceiver), and function as a gateway or bridge between a WLAN and a wired LAN.
A block diagram illustrating an example wireless communications system is shown in FIG. 1. The example system, generally referenced 10, comprises one or more mobile devices 12 implementing a WLAN station (or STA) connected to access point (AP) 14 which is connected to WLAN 16. The system also comprises a cellular base station 20 in communication over an air interface to mobile device 12. Base station 20 is connected to a WWAN 22 (e.g. a cellular telecommunications network) which is also connected to WLAN 16.
In implementing the WLAN protocol, mobile devices often utilize so called WLAN profiles to aid in establishing connections with the APs and WLANs. A WLAN profile defines the parameters for the connection between the mobile device and WLAN networks including APs. Profiles typically include connection-related information including, for example, Set Service Identifier (SSID), connection type (i.e. open or shared key), security, authentication, encryption, WEP shared keys, key length, frequency bands, roaming enable/disable, SSID broadcasted, etc.
A SSID identifies a particular WLAN. Mobile device 12 receives broadcast messages from all access points within range advertising their SSIDs. Mobile device 12 can then either manually or automatically select the network with which to associate. It is legitimate for multiple APs to share the same SSID if they provide access to the same network as part of an extended service set. On the other hand, the BSS is the basic building block of an IEEE 802.11 wireless LAN. In infrastructure mode, one AP together with all associated mobile devices is called a BSS. An AP acts as a master to control the mobile devices within that BSS. Each BSS is identified by a BSS Identifier (BSSID). The most basic BSS is two mobile devices in Independent mode. In infrastructure mode, a basic BSS consists of one AP and one mobile device. The BSSID uniquely identifies each BSS (the SSID however, can be used in multiple, possibly overlapping, BSSs). In an infrastructure BSS, the BSSID is the MAC address of the wireless access point (WAP).
When a mobile device wants to access an existing BSS (for example, as after power-up, sleep mode or just entering a BSS area), the mobile device must get synchronization information from the AP. The mobile device obtains this information by either (1) passive scanning whereby the mobile device waits to receive a beacon frame (and/or probe responses sent in response to other devices' probe requests) from the AP; or (2) active scanning whereby the mobile device attempts to find an AP by transmitting probe request frames and waiting for a probe response from the AP. Note that the beacon frame is a periodic frame sent by the AP containing synchronization information. Once the mobile device has found an AP, in order to join the BSS, it must perform an authentication process which involves the exchange of information between the AP and the mobile device, where each side shows knowledge of a shared credential(s). Once authenticated, the mobile device begins the association process which involves the exchange of information about the mobile device and BSS capabilities. Only after the association process is complete, is the mobile device permitted to transmit and receive data frames with the AP.
As mentioned above, the mobile device may be configured to operate for communications in WLANs as well as a cellular telecommunication network, more generally referred to herein as one type of wireless wide area networks (WWAN). With these types of mobile devices, it is advantageous to be able to quickly and reliably detect when the mobile device should be handed over from the WLAN to the WWAN, and vice versa, so that communication sessions (e.g. voice calls or data calls) of the mobile device may be seamlessly maintained. Handing over from one type of wireless network to another type of wireless network (i.e. handing over between such heterogeneous networks) may be referred to as a vertical handover (VHO). If the mobile device is able to quickly and reliably detect its movement or transitioning (e.g. that the mobile device is about to leave the WLAN), for example, a connection may be established between the mobile device and the WWAN before the connection with the WLAN is lost.